Fiber reinforced polymeric matrix composites are being used in many structural applications such as aircraft, aerospace, automotive, and sporting goods applications. Generally, these high performance composites contain orientated continuous carbon fibers cured in a matrix resin. Glass or Kevlar® fibers may also be used in these applications. While there are many types of matrices that find use in composite applications, epoxy resins have dominated the market due to ease of use, excellent properties, and relatively low cost.
More recently, because of their excellent physical and mechanical properties and high thermal stability, polybenzoxazine resins have been used in producing high performance composites. Such polybenzoxazine resins can be obtained from the reaction of polyhydric phenols, formaldehyde and an amine as described in U.S. Pat. Nos. 4,607,091, 5,152,993, 5,266,695 and 5,543,516. One drawback to such polybenzoxazine resins is they are solid at room temperature and must be melted when used in the manufacture of composites, for example, in resin transfer molding (RTM) and vacuum assisted resin transfer molding (VaRTM) manufacturing processes.
In RTM and VaRTM, a fiber preform is fabricated in the shape of a finished composite article then placed in a closed cavity mold. A resin is then injected into the mold to initially wet and eventually impregnate the preform. In RTM, the resin is injected under pressure into the mold and then cured to produce the composite. In VaRTM, the preform is covered by a flexible sheet or liner which is clamped onto the mold to seal the preform in an envelope. The resin is then introduced into the envelope to wet the preform and a vacuum is applied to the interior of the envelope via a vacuum line to collapse the flexible sheet against the preform and draw the resin through the preform. The resin is then cured while being subjected to vacuum.
Thus, in both RTM and VaRTM systems, the resin which is used must possess a very low injection viscosity to allow for complete wetting and impregnation of the preform. Further, the resin must maintain such low viscosity for a period of time sufficient to completely fill the mold and impregnate the fiber preform. Finally, the resin must be homogeneous before cure. These requirements limit the use of polybenzoxazine resins in RTM and VaTRM systems since, upon melting, they impart a higher than desired viscosity, contain solids in particulate form and quickly return to a solid form at temperatures below about 100° C.
WO 2000/61650 describes the preparation of a liquid benzoxazine resin from a monohydric phenol, aldehyde and amine and its use in applications where it's necessary to apply a benzoxazine resin as a liquid at room temperature. However, this liquid benzoxazine resin still exhibits a higher than desired viscosity at room temperature and a short pot life making its use in RTM and VaRTM systems challenging.